Surface Enhanced Raman Spectroscopy Experiments Research Articles

Electro-inductive Effect Dominates Vibrational Frequency Shifts of Conjugated Probes on Gold Electrodes.

Interfacial electric fields play a critical role in electrocatalysis and are often characterized by using vibrational probes attached to an electrode surface. Understanding the physical principles dictating the impact of the applied electrode potential on the vibrational probe frequency is important. Herein, a comparative study is performed for two molecular probes attached to a gold electrode. Both probes contain a nitrile (CN) group, but 4-mercaptobenzonitrile (4-MBN) exhibits continuous conjugation from the electrode through the nitrile group, whereas this conjugation is interrupted for 2-(4-mercaptophenyl)acetonitrile (4-MPCN). Periodic density functional theory calculations predict that the CN vibrational frequency shift of the 4-MBN system is dominated by induction, which is a through-bond polarization effect, leading to a strong potential dependence that does not depend significantly on the orientation of the CN bond relative to the surface. In contrast, the CN vibrational frequency shift of the 4-MPCN system is influenced less by induction and more by through-space electric field effects, leading to a weaker potential dependence and a greater orientation dependence. These theoretical predictions were confirmed by surface-enhanced Raman spectroscopy experiments. Balancing through-bond and through-space electrostatic effects may assist in the fundamental understanding and design of electrocatalytic systems.

Detection of Alternative Drugs for Illegal Injection Based on Surface-Enhanced Raman Spectroscopy

Promethazine and scopolamine are two major components of a drug commonly used as a substitute for heroin addiction but with its own potential for abuse. Here, surface-enhanced Raman spectroscopy experiments were combined with density functional theory computations of the Raman frequencies of promethazine. The enhancement of the SERS substrate was optimized by contrast detection of silver nanoparticles. The optimized substrate was used to detect scopolamine, promethazine and their mixtures. Several mixtures of different concentrations were detected and analyzed. The approach of rapid identification and monitoring of drugs will become an important tool in the detection and control of illicit drug use.

Nonlinear inelastic electron scattering from Au nanostructures induced by localized surface plasmon resonance

Nonlinear electron scattering is a recently-discovered physical process observed during the localized plasmonic excitation of Ag nanostructures on graphite surface. In the present work, nonlinear electron scattering phenomena is experimentally verified on Au nanostructures by measuring inelastic scattering of electrons field-emitted from tungsten tip. The relative intensity of the electron-energy-loss peak associated with the plasmonic excitation of Au shows again to increase nonlinearly with the electric field generated by the tip-sample bias, demonstrating the generality of nonlinear electron scattering process in plasmonic system. Compared to the nonlinear electron scattering phenomena observed on Ag nanostructures, the nonlinear term for Au nanostructures is about 1 to 2 orders of magnitude smaller, which is in consistent with the field enhancement factor of Au and Ag nanostructures from both the surface-enhanced Raman spectroscopy experiments and the theoretical calculations.

Microsphere Lithography on Hydrophobic Surfaces for Generating Gold Films that Exhibit Infrared Localized Surface Plasmon Resonances

Evaporation induced self-assembly is an established method for producing close-packed two-dimensional sphere masks on hydrophilic surfaces such as glass. In sphere lithography, gold or silver is deposited over sphere masks to generate a film-over-nanospheres or a nanoprism array that can be used as a sensing surface in localized surface plasmon extinction and surface enhanced Raman spectroscopy experiments. Sphere lithography is less commonly used to prepare sphere masks on hydrophobic surfaces associated with infrared window materials, in part because it is challenging to find solvents with wetting and evaporation characteristics that are appropriate for such surfaces. This wetting challenge can be overcome with appropriate surfactants. However, surfactant residues are then left behind on the sensing surface. We report methods for depositing monolayer crystalline sphere masks onto CaF2 windows that minimize surfactant residue by either using ethanol as a volatile cosolvent that enhances wetting, or by increasing the concentration of colloid to compensate for the reduced attractions between spheres and surface. The rate of evaporation of solvents from the colloid drop is controlled by fixing the headspace partial pressure of ethanol and/or water.

Controlled Growth and Positioning of Metal Nanoparticles via Scanning Probe Microscopy

A process enabling both the controlled growth and positioning of metal nanoparticles (NPs) is reported. Using scanning probe microscopy (SPM) techniques, metal NPs are directly grown in the region of interest via the reduction of metallic ions in a polymer matrix induced by a properly biased SPM tip. The metallic nature of these NPs is established via X-ray diffraction and surface-enhanced Raman spectroscopy experiments. Some initial applications of this process, such as the decoration of carbon nanotubes with metal NPs, are also briefly demonstrated and discussed.

In-situ SERS experiments during the electrodeposition of gold in the presence of benzyldimethylphenylammonium chloride

Hydrogen incorporation is one of the main problems encountered in electroforming and electroplating processes and can result in serious material embrittlement. Use of organic additives has been shown to reduce hydrogen incorporation efficiently in electrodeposition of gold from neutral cyano–aurate baths. We report the results of: (i) in-situ surface enhanced Raman spectroscopy experiments; supported by (ii) electrochemical measurements (cyclic voltammetry, differential capacitance); and (iii) GC–MS analyses, relevant to electrodeposition of gold from a KAu(CN) 2 aqueous solution, containing benzyldimethylphenylammonium chloride (BDMPAC). The cathodic coadsorption of cyanide and the quaternary ammonium salt causes a remarkable depolarisation of the metal deposition. The results obtained match well with the hypothesis of two surface reactions, taking place at potentials more negative than −1300 mV versus Ag ∣ AgCl, which involve the additive and bring about the consumption of both hydrogen and cyanide adsorbed on the electrode. Depolarising effects and reduced hydrogen incorporation accomplished with the use of BDMPAC can thus be related to its hydrogen and cyanide scavenging action.